1. Field of Invention
This invention pertains to method and apparatus for detecting particles, particularly to method and apparatus for detecting particles (such as neutrons) in a manner to discriminate between the particles and background radiation (such as gamma rays).
2. Prior Art and Other Considerations
It is well known that, in general, neutron counters not only respond to neutrons but to gamma rays as well. These gamma rays can be considered as background counts and hence, contribute to the experimental uncertainty of the parameter that is being measured, assuming the parameter being measured depends on the number of neurons that are counted. Hence, it is important to develop techniques to differentiate the neutron counts from the background gamma ray counts.
Neutron-gamma ray discrimination has been accomplished in the past by pulse shape discrimination. See, for example, P. Sperr, H. Spieler and M. R. Maier, "A Simple Pulse Shape Discrimination Circuit", Nuc. Instr. and Meth., 116 (1974), pp. 55-59. This technique is made possible due to the fact that the pulse shapes of certain scintillators depend on the exciting particle. For instance, the organic scintillator, stilbene, gives a different pulse shape out when excited by neutrons than when excited by gamma rays. This is due to the fact that the scintillator is excited by recoil protons when excited by neutrons and is excited by Compton generated electrons when excited by gamma rays. Other scintillators that exhibit pulse shape scintillation are anthracene and certain liquid scintillators. See Miller, Thomas G., "Measurement of Pulse Shape Discrimination Parameters for Several Scintillators", Nuc. Instr and Meth., 63 (1968), pp. 121-122.
Present day neutron-gamma ray discrimination circuits are limited in their usefulness due to their count rate limitation. The maximum count rate for neutron-gamma ray separation using present techniques is limited due to pulse pile-up in the scintillator itself, due to the long component of the neutron pulse; and, due to the problem of pulse processing in the electronics. Progress has been made in recent years in building neutron-gamma ray discrimination circuits that achieve higher count rates, but it appears that about 50,000 counts per second may be an upper limit if good resolution is to be achieved.
Another problem with present day neutron-gamma ray discrimination circuits is that they achieve good separation only in a limited neutron energy range which is about 1 MeV to perhaps 14 Mev.
A fiber optics neutron detector was proposed by Thomas G. Miller, Welman Gebhart, Lee Hilbert, and George Edlin (see "Fiber Optics Neutron Detectors", presented at the Workshop on Scintillating Fiber Detector Development for the SSC, Nov. 14-16, 1988). According to this proposal, an optical fiber bundle is aimed at a neutron source so that the neutrons are incident on the end of the fibers (e.g., along the axis of the fibers). An opposite end of the optical fiber bundle (e.g., the non-incident end) is flush against an intensifier, which in turn is positioned against a detector. The optical fibers have square cross section and measure 300 microns by 300 microns. The number of fibers traversed by a neutron is used as an indication of the directionality of an incoming neutron beam.
The optical fiber neutron detector discussed in the preceding paragraph requires a specialized detector, such as a charge coupled device (CCD). Moreover, data reduction of the CCD signals is rather formidable, requiring expensive and sophisticated electronics and software.
It is well known that background gamma rays interact with scintillating material to produce Compton electrons. As the Compton electrons travel through scintillating material, the electrons slow down and lose energy via ionization processes which cause emission of photons (e.g., light pulses). The distance a Compton electron can travel in the scintillating material is a function of the electron energy. As shown in FIG. 1, the range of typical electrons produced by the background gamma rays in the scintillating material will be several thousand microns.
When neutrons interact with scintillating material, on the other hand, recoil protons are created. As the recoil protons travel through the scintillating material, the recoil protons also slow down and lose energy via ionization processes which result in the emission of photons. As shown in FIG. 2, the range of typical recoil protons produced by neutrons will have ranges in the scintillating material which are considerably less than the distance ranges of electrons. For example, recoil protons in the range of from 0.5 MeV to 3.0 MeV can travel a maximum distance of about 100 microns.
In view of the foregoing concerns regarding the prior art, it is an object of the present invention to provide neutron/gamma ray discrimination apparatus and method having high count rate and good resolution.